Examples of a method of starting a motor include a direct on line starting method and a star-delta soft starter method. The direct on line starting method is a method that directly supplies a source voltage to a motor by using an electronic switch, and the star-delta soft starter method is a method that, by using three electronic switches, converts a wiring of an induction motor into a star wiring type in starting, and converts the wiring of the induction motor into a delta wiring type in driving.
Generally, in a motor starter that starts a motor, the motor starter or an electronic motor protection relay (EMPR) is not singly used, and the motor starter is equipped in a panel (a motor control center (MCC)) in a type of a unit rack, and is used. Therefore, a wiring operation of connecting the elements is needed.
FIG. 1 illustrates a configuration of a general motor starter apparatus. As illustrated in FIG. 1, the motor starter apparatus includes a motor starter 1 in which an electromagnetic contactor and a motor protection circuit breaker are provided in a type of a unit rack, a PLC 3 that receives a user control command or processes and outputs an input signal, and a terminal block port 5. An automatic/manual selector and a software program for local control are installed in the motor starter apparatus, and the motor starter apparatus includes a plurality of relays and a plurality of wirings for controlling the electromagnetic contactor.
An operation of the motor starter apparatus will be described below. First, in order to start a motor, an on/off state of the electromagnetic contactor should be controlled by using an input/output (I/O) of the PLC 3. To this end, an analog input module of the PLC 3 receives an analog output value (for example, an output value of 4 mA to 20 mA) from an external sensor. The PLC 3 performs a program operation of controlling the on/off state of the electromagnetic contactor by using the received analog input value.
In this case, in order to control the electromagnetic contactor, the PLC 3 should know current state information of the electromagnetic contactor, and thus, an auxiliary contact should be installed in the electromagnetic contactor, and should be connected to a state processing module of the PLC 3 by wire. Also, the PLC 3 should periodically sense a state (an on or off state) of the electromagnetic contactor. Thus, an auxiliary relay for controlling the electromagnetic contactor is installed in the PLC 3, and the electromagnetic contactor is controlled by a control processing module of the PLC 3.
As described above, since the PLC 3 recognizes and processes a state of the electromagnetic contactor, many wiring operations are required for controlling the electromagnetic contactor, and moreover, a problem of securing the cost and a space is caused. Furthermore, when the analog input module or state processing module of the PLC 3 are broken down, it is unable to control an on/off state of the electromagnetic contactor, and for this reason, the motor starter cannot operate.